Protein B23 is a nucleolar phosphoprotein found in higher concentrations in tumor cells than in normal cells. This protein is associated with pre- ribosomal particles and is localized in the granular region of nucleoli where ribosomes are assembled. When treated with antitumor drugs (Actinomycin D., Doxorubicin, Luzopeptins, Mitomycin C or Toyocamycin), protein B23 translocates from the nucleolus to the nucleoplasm. This result indicates that these antitumor drugs affect the protein B23 binding target in the nucleolus. The proposed experiments are designed to (1) identify the binding target of protein B23 in the nucleolus which is affected by the antitumor drugs and analyze this binding site's structure (nucleotide and amino acid sequences) which will provide information for new drug development, (2) understand the function of protein B23 (its role in ribosome synthesis of tumor cells), and (3) establish a rapid and simple immunofluorescence method using the protein B23 antibody to detect drug resistant cells, to screen antitumor drugs, and monitor drug efficacy. Our long term goals are to understand the differences between the ribosome maturation processes in normal and cancer cells, to study the effects of antitumor drugs on ribosome synthesis and maturation, and to provide information for development of new inhibitors or competitors for ribosome synthesis which could be helpful in chemotherapy.